RESUMO
Cells of almost any organism respond to a sudden up-shift of temperature and to several other stress conditions, by a transient increase in the cellular concentration of a set of proteins, the heat-shock proteins (HSPs). The main HSPs, chaperones and proteases, are constituents of the cellular machinery of protein folding, translocation, repair and degradation. The bacteria Escherichia coli has been a paradigm regarding heat shock gene expression in prokaryotes. In this bacterium, the expression of the HSPs is regulated at the transcriptional level. The approximately 40 genes that encode the HSPs define the heat-shock stimulon. Most of these genes, including the main chaperone and protease genes, are under the positive control of sigma32, encoded by rpoH, while approximately 10 genes, including rpoH and rpoE, are regulated by sigma(E) , encoded by rpoE. The cytoplasmic response to heat is regulated by sigma32, while that of the periplasm is regulated by sigma(E). The expression of both regulons is interconnected, since sigma(E) regulates the transcription of rpoH at high temperatures. The activity of these sigma factors, under non-stress and stress conditions, depends upon negative and positive regulatory mechanisms acting at different levels: transcription, translation, half-life and activity of the factors. Models for the regulation of the cytoplasmic and periplasmic response to heat in E. coli are presented.